Commonly assigned application Ser. No. 751,530, filed July 3, 1985, now U.S. Pat. No. 4,733,718, discloses a process in which heat exchangers comprising a bundle of stacked, extruded, plastic tubular plates are face welded into a connected bundle by means of strip heaters placed between the hollow chambers. While satisfactory in many situations, this procedure is difficult to apply to objects with a large internal cavity surrounded by an outer wall--such as, for example, pipes.
German published patent application DE-PS No. 2 603 615 describes a process for welding bundles of thermoplastic pipes wherein each bundle is clamped between two heated plates which are pressed against the ends of the bundle. The ends of the bundle have annular grooves matching the pipes. The ends of the pipes that extend into the grooves are melted and welded together. During this process, however, only negligible pressure can be applied to the bundle because otherwise the melt would be forced out of the grooves, the width of which is larger than the wall thickness of the pipes. The negligible pressure that can be applied, however, does not guarantee that the ends of the pipes will be faultlessly welded together.